DFS: When, Why, and How
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DFS: When, Why, and How

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• Relates to: MCSE 2000 | MCSA 2000

Distributed File System (DFS) is a logical, hierarchical file
system that combines resources from across a site (or multiple
sites) into a single namespace. That is, a single share point on
your network can be used to provide access to resources located
on different servers. Just as a file system is used to provide
uniform, named access to your local hard disk, DFS provides a
uniform naming convention and mapping for groups of servers,
shares, and files. Because DFS can be used to build a
hierarchical file structure whose contents are distributed
throughout the organizational WAN, DFS can be thought of as a
share of other shares.

For example, the staff in your organization uses resources on
five separate servers, BRENNER, CRAMER, GOODWIN, HORSTMAN, and
WOLFE:

\\BRENNER\APPS
\\CRAMER\RESEARCH
\\GOODWIN\USERS
\\HORSTMAN\RECORDS
\\WOLFE\PAYROLL

With a small number of shares most network administrators will
map drive letters to shares rather than expect users to remember
and type in the UNC path of each resource. However in many larger
networks, there may be no drive letters left or an ever-
increasing number of shares renders this impractical. Enter DFS.
By organizing all of these disparate shares under a new virtual
root, all resources will now appear under a central share point
that can be mapped to a single drive letter. When the virtual (or
logical) share is accessed, the client system is invisibly
redirected to the physical resource without the user needing to
be aware of what happened.


Differences in DFS between NT4 and W2K:

Microsoft originally released DFS with Windows NT 4.0. Although
NT4 shipped with the client portion of DFS, the server portion
(DFS version 4.1) had to be downloaded and installed separately.
Once the server side was installed, an administrator suddenly had
the ability to place shares from Windows 95, Windows 98, Windows
NT, and even NetWare boxes throughout the organizational WAN
under a single easy-to-browse tree.

There was one big problem, however. If the NTServer that was
hosting the DFS virtual root went offline, users could no longer
access the resources they needed through it. The resources were
still there, but unless users knew the specific location of each
resource and could locate it manually, they were out of luck.
Even worse, there was no easy way to provide fault-tolerance for
the shared resources themselves in the event they went off-line
(Directory Replication was not suitable for large files and could
quickly congest networks with high traffic). The lack of fault-
tolerance for DFS under Windows NT 4.0 hindered widespread
adoption of the technology.

This has all changed with DFS version 5 for Windows 2000.
Microsoft has made DFS completely fault-tolerant by building it
on top of the new Windows File Replication Service (FRS) and
integrating it with Active Directory (AD). Now a virtual DFS root
can span multiple servers, providing fault-tolerance and
eliminating the single point-of-failure issues that plagued DFS
under NT4.

The shared resources themselves can now also be replicated to
increase their availability. The Directory Replication Service
(DRS) in NT4 was completely unsuitable for moving around large
amounts of data. In Windows 2000, DRS has been discarded (it is
only used for backwards compatibility in mixed-mode domains) in
favour of the FRS which can be configured through AD. Replication
of critical data can be scheduled to avoid congesting the local
LAN or slow WAN links.

Finally, under NT4 DFS, you were only allowed a single-level file
hierarchy. Using a domain-based Windows 2000 Distributed File
System allows your DFS root to have a hierarchy depth limited
only by the 260-character file path name restriction in Windows.


Creating the DFS Root:

On your W2K Server, click Start > Programs > Administrative Tools
> Distributed File System. In the left-hand pane, right-click on
"Distributed File System" and choose "New DFS Root Volume". A new
dialog box will appear asking you to choose whether you want to
"Create a domain DFS root" or "Create a stand-alone DFS root".

A domain DFS root is published in Active Directory and must be
hosted on a Domain Controller (DC). If you want complete fault-
tolerance (through replication) and the ability to create deep
file and folder hierarchies, choose this option. Stand-alone DFS
suffers the same limitations under W2K as DFS under NT4. It is
not fault-tolerant, cannot be replicated, and...

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